When you run WinCron as a system service, it is likely running in a different account from the currently logged-in user.
This isn’t always obvious because when you run WinCron in the LocalSystem Account, you can see the User Interface (console window) so you would assume it is running within the context of currently logged in user.

When WinCron—or any service for that matter—runs in the LocalSystem account, WinCron only has access to the Environment variables, Network drives, etc., that have been configured for the LocalSystem.
Most times, the LocalSystem has no network access and the environment variables are minimal . (Your normal PATH information is unavailable.)

The easiest way to configure the network for the LocalSystem is with NET.EXE.
NET.EXE is a program that allows you to configure many aspects of your computer, including network access.

The particular NET command to map a drive is NET USE params
Here is the help for net use:
C:\>net help use
NET USE [devicename | *] [\\computername\sharename[\volume] [password | *]]
[/USER:[domainname\]username]
[/USER:[dotted domain name\]username]
[/USER:[username@dotted domain name]
[[/DELETE] | [/PERSISTENT:{YES | NO}]]

NET USE {devicename | *} [password | *] /HOME

NET USE [/PERSISTENT:{YES | NO}]


NET USE connects a computer to a shared resource or disconnects a
computer from a shared resource. When used without options, it lists
the computer's connections.

devicename Assigns a name to connect to the resource or specifies
the device to be disconnected. There are two kinds of
devicenames: disk drives (D: through Z and printers
(LPT1: through LPT3 . Type an asterisk instead of a
specific devicename to assign the next available
devicename.
\\computername Is the name of the computer controlling the shared
resource. If the computername contains blank characters,
enclose the double backslash (\\) and the computername
in quotation marks (" "). The computername may be from
1 to 15 characters long.
\sharename Is the network name of the shared resource.
\volume Specifies a NetWare volume on the server. You must have
Client Services for Netware (Windows Workstations)
or Gateway Service for Netware (Windows Server)
installed and running to connect to NetWare servers.
password Is the password needed to access the shared resource.
* Produces a prompt for the password. The password is
not displayed when you type it at the password prompt.
/USER Specifies a different username with which the connection
is made.
domainname Specifies another domain. If domain is omitted,
the current logged on domain is used.
username Specifies the username with which to logon.
/HOME Connects a user to their home directory.
/DELETE Cancels a network connection and removes the connection
from the list of persistent connections.
/PERSISTENT Controls the use of persistent network connections.
The default is the setting used last.
YES Saves connections as they are made, and restores
them at next logon.
NO Does not save the connection being made or subsequent
connections; existing connections will be restored at
next logon. Use the /DELETE switch to remove
persistent connections.
NET HELP command | MORE displays Help one screen at a time.

So, to map a network drive G: to computer \\main on volume \imgstore, I would do the following:
net use g: \\main\imgstore my_password /USER:my_username

You can place these commands in a batch file, or call them directly from a WinCron script.

SUGGESTION: a good way to debug this scenario is to write a run-once script that simply launches CMD.EXE (this must be done while WinCron is running as a service.)
The CMD window that opens will be a portal into the LocalSystem account whereby you can do things like check for network connectivity and environment settings. Type SET to get a dump of the environment variables available to the LocalSystem account. You can modify these variables in the Control Panel/Settings/Advanced/Environment Variables/System Variables (This is for Win2k, NT is similar.)

You may want to also try mapping your network drive in this CMD window to get the syntax right before committing the command to a WinCron script.
As mentioned above, you should be able to enter something like:
net use g: \\main\imgstore my_password /USER:my_username
to map a drive. There are also other options to NET USE that you may want to consider.